This invention relates to the preparation of lightly tinted glasses of variable transmission which lend themselves for ophthalmic lens applications.
In the present context, it is understood that glasses "lightly tinted of variable transmission" are glasses exhibiting a light tint (for example, gray, gray-brown, brown, pink) and having a luminous transmission which changes very rapidly and in a reversible manner as a function of the exposure of the glass to a source of actinic radiation, but relatively little when compared to conventional photochromic glasses since their luminous transmission before exposure is located within the range of 60-80%.
The majority of patents concerning photochromic glasses describe compositions permitting products for ophthalmic applications to be obtained which are capable of achieving, through darkening, a range of transmission between 10-40% at ambient temperature (20.degree.-25.degree. C.).
The progress realized permits us today to satisfy the needs, with respect to solar protection, of many wearers of ophthalmic lenses. In fact, the presence in the marketplace of photochromic glasses, distinct products capable of covering the levels of darkening between 10-60%, gives to the users a freedom of choice adapted to their specific needs.
There exists, however, a large category of ophthalmic lens wearers, namely, wearers of glasses wherein the transmission does not vary, designated "fixed tints." These people do not want glasses exhibiting a luminous transmission below about 60% and, preferably, not below about 70% ("fixed tints" A and A-B).